Recently, a light emitting device using light emitting elements such as light emitting diodes (LEDs) is used as a backlight of a liquid crystal panel and lighting equipment. FIG. 1 is a circuit diagram illustrating a configuration of a light emitting device 1003. The light emitting device 1003 and its operation to be described below are not considered as conventional technology by the present applicant. The light emitting device 1003 includes an LED string 6, a switching power supply 1004 and a current driver circuit 1008.
The LED string 6 includes a plurality of LEDs which are connected in series. The switching power supply 1004 steps up an input voltage VIN to supply a drive voltage VOUT to one end of an anode side of the LED string 6.
The current driver circuit 1008 adjusts luminance of the LED string 6 using a combination of analog dimming and burst dimming (also referred to as PWM dimming). A current source CS is connected to one end of a cathode side of the LED string 6 to supply a drive current ILED to the LED string 6 according to target luminance. The dimming based on a magnitude of the drive current ILED is referred to as analog dimming.
A PWM controller 1009 intermittently turns on the current source CS at a duty ratio according to the burst dimming. Accordingly, the drive current ILED flows through the LED string 6 only during an ON period TON according to the duty ratio, so that the time average of the drive current ILED is controlled and the luminance is adjusted.
The switching power supply 1004 includes an output circuit 1102 and a control integrated circuit (IC) 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1 and an output capacitor C1. The control IC 1100 adjusts the drive voltage VOUT by controlling a duty ratio of ON/OFF of the switching transistor M1.
The control IC 1100 stabilizes the drive voltage VOUT such that a voltage between both ends of the current source CS. That is, a potential (referred to as a detection voltage) VLED of one end of the cathode side of the LED string 6 is equal to a reference voltage VREF. An error amplifier 22 amplifies an error of the detection voltage VLED and the reference voltage VREF, and generates a feedback voltage VFB. A pulse width modulator PWM generates a pulse signal SPWM having a duty ratio according to the feedback voltage VFB. A driver DR drives the switching transistor M1 based on the pulse signal SPWM.
The drive voltage VOUT is divided by resistors Ro1 and Ro2, and input to the control IC 1100. The control IC 1100 detects an over-voltage state or the like by using the divided drive voltage VOUT (hereinafter referred to as OVP voltage VOVP).
A standby signal STB is inputted to the control IC 1100 from a microcomputer. The control IC 1100 turns on the LED string 6 by the above-mentioned operation when the standby signal STB reaches a first level (e.g., high level), and turns off the LED string 6 by turning off the current source CS while stopping the switching transistor M1 when the standby signal STB becomes a second level (e.g., low level).
FIG. 2 is a waveform diagram illustrating an operation of the light emitting device 1003 of FIG. 1. Prior to a time t1, the standby signal STB is set at a high level. At this time, the output voltage VOUT is stabilized to a predetermined level, and the drive current ILED flows in the LED string 6. Further, the feedback voltage VFB is also kept in the vicinity of a certain voltage level Va. If the detection voltage VLED is larger than the reference voltage VREF, the feedback voltage VFB is lowered and the ON period of the switching transistor M1 is shortened, so that feedback is applied to reduce the drive voltage VOUT. On the other hand, if the detection voltage VLED is smaller than the reference voltage VREF, the feedback voltage VFB increases, so that feedback is applied to shorten the ON time of the switching transistor M1.
At the time t1, the standby signal STB steps down to a low level. Accordingly, internal circuit blocks of the control IC 1100, such as a PWM (pulse width modulator) 20, the error amplifier 22 and a driver 28, are shut down, and switching of the switching transistor M1 is stopped. Further, if the standby signal STB is set at a low level, applying of the current source CS to the light emitting device 1003 is also stopped.
If the switching of the switching transistor M1 is stopped, charges of the output capacitor C1 are discharged through the resistors Ro1 and Ro2, and the drive voltage VOUT is reduced slowly. Further, the feedback voltage VFB output from the error amplifier 22 is reduced substantially to a ground voltage of 0 V by the shutdown of the error amplifier 22.
At time t2, the standby signal STB steps up to a high level. Accordingly, the control IC 1100 returns to an operation state from a standby state, and the operation of the internal circuit blocks is resumed. If a standby period TSTB is not too long, as the drive voltage VOUT at the time t2 maintains a sufficiently high level, the drive current ILED flows in the LED string 6 to emit light.
At the time t2, the drive voltage VOUT is lower than a target value, and the detection voltage VLED is lower than the reference voltage VREF. Accordingly, in order to increase the detection voltage VLED, it is necessary to apply feedback to lengthen the ON time of the switching transistor M1. However, since the feedback voltage VFB is reduced to 0 V, although the detection voltage VLED is lower than the reference voltage VREF, the ON time of the switching transistor M1 becomes shorter, so that feedback is applied to further reduce the detection voltage VLED. Accordingly, the drive current ILED is reduced. Then, the feedback voltage VFB approaches the original voltage level Va, and the detection voltage VLED also returns to its original level.
As described above, in the light emitting device 1003 of FIG. 1, when returning from the standby state, the LED string 6 emits light, and after light emission intensity is reduced, the LED string 6 emits light at target intensity. This is undesirable because it appears as flickering of the LED string 6.